regexec(3)
regcomp, regexec, regerror, regfree – POSIX regex functions
Synopsis
#include <regex.h>
int regcomp(regex_t *preg, const char *regex, int cflags);
int regexec(const regex_t *preg, const char *string, size_t nmatch,
regmatch_t pmatch[], int eflags);
size_t regerror(int errcode, const regex_t *preg, char *errbuf,
size_t errbuf_size);
void regfree(regex_t *preg);
Description
POSIX regex compiling
regcomp() is used to compile a regular expression into a form that is suitable for subsequent regexec() searches. regcomp() is supplied with preg, a pointer to a pattern buffer storage area; regex, a pointer to the null-terminated string and cflags, flags used to determine the type of compilation. All regular expression searching must be done via a compiled pattern buffer, thus regexec() must always be supplied with the address of a regcomp() initialized pattern buffer. cflags
is the
bitwise-or
of zero or more of the following:
REG_EXTENDED
Use
POSIX
Extended Regular Expression syntax when interpreting
regex.
If not set,
POSIX
Basic Regular Expression syntax is used.
REG_ICASE
Do not differentiate case.
Subsequent
regexec()
searches using this pattern buffer will be case insensitive.
REG_NOSUB
Do not report position of matches.
The
nmatch
and
pmatch
arguments to
regexec()
are ignored if the pattern buffer supplied was compiled with this flag set.
REG_NEWLINE
Match-any-character operators don’t match a newline.
A nonmatching list
([ha…])
not containing a newline does not match a newline.
Match-beginning-of-line operator
(ha)
matches the empty string immediately after a newline, regardless of
whether
eflags,
the execution flags of
regexec(),
contains
REG_NOTBOL.
Match-end-of-line operator
($)
matches the empty string immediately before a newline, regardless of
whether
eflags
contains
REG_NOTEOL.
POSIX regex matching
regexec()
is used to match a null-terminated string
against the precompiled pattern buffer,
preg.
nmatch
and
pmatch
are used to provide information regarding the location of any matches.
eflags
is the
bitwise-or
of zero or more of the following flags:
REG_NOTBOL
The match-beginning-of-line operator always fails to match (but see the
compilation flag
REG_NEWLINE
above).
This flag may be used when different portions of a string are passed to
regexec()
and the beginning of the string should not be interpreted as the
beginning of the line.
REG_NOTEOL
The match-end-of-line operator always fails to match (but see the
compilation flag
REG_NEWLINE
above).
REG_STARTEND
Use
pmatch[0]
on the input string, starting at byte
pmatch[0].rm_so
and ending before byte
pmatch[0].rm_eo.
This allows matching embedded NUL bytes
and avoids a
strlen(3)
on large strings.
It does not use
nmatch
on input, and does not change
REG_NOTBOL
or
REG_NEWLINE
processing.
This flag is a BSD extension, not present in POSIX.
Byte offsets
Unless
REG_NOSUB
was set for the compilation of the pattern buffer, it is possible to
obtain match addressing information.
pmatch
must be dimensioned to have at least
nmatch
elements.
These are filled in by
regexec()
with substring match addresses.
The offsets of the subexpression starting at the
ith
open parenthesis are stored in
pmatch[i].
The entire regular expression’s match addresses are stored in
pmatch[0].
(Note that to return the offsets of
N
subexpression matches,
nmatch
must be at least
N+1.)
Any unused structure elements will contain the value -1. The regmatch_t structure which is the type of pmatch is defined in <regex.h>. typedef struct {
POSIX error reporting
regerror() is used to turn the error codes that can be returned by both regcomp() and regexec() into error message strings. regerror() is passed the error code, errcode, the pattern buffer, preg, a pointer to a character string buffer, errbuf, and the size of the string buffer, errbuf_size. It returns the size of the errbuf required to contain the null-terminated error message string. If both errbuf and errbuf_size are nonzero, errbuf is filled in with the first errbuf_size – 1 characters of the error message and a terminating null byte (‘